Allergies are diseases of the immune system that cause an overreaction to substances called "allergens." Allergies are grouped by the kind of trigger, time of year or where symptoms appear on the body:indoor andoutdoorallergies (also called "hay fever," "seasonal," "perennial" or "nasal" allergies),foodallergies,latex allergies,insectallergies,skinallergies andeyeallergies. People who have allergies can live healthy and active lives.

The substances that cause allergic disease in people are known as allergens. “Antigens,” or protein particles like pollen, food or dander enter our bodies through a variety of ways. If the antigen causes an allergic reaction, that particle is considered an “allergen” – and antigen that triggers an allergic reaction. These allergens can get into our body in several ways:

Inhaled into the nose and the lungs. Examples are airborne pollens of certain trees, grasses and weeds; house dust that include dust mite particles, mold spores, cat and dog dander and latex dust.

Ingested by mouth. Frequent culprits include shrimp, peanuts and other nuts.

Injected. Such as medications delivered by needle like penicillin or other injectable drugs, and venom from insect stings and bites.

Absorbed through the skin. Plants such as poison ivy, sumac and oak and latex are examples.

What Makes Some Pollen Cause Allergies, and Not Others?

Plant pollens that are carried by the wind cause most allergies of the nose, eyes and lungs. These plants (including certain weeds, trees and grasses) are natural pollutants produced at various times of the year when their small, inconspicuous flowers discharge literally billions of pollen particles.

Because the particles can be carried significant distances, it is important for you not only to understand local environmental conditions, but also conditions over the broader area of the state or region in which you live. Unlike the wind-pollinated plants, conspicuous wild flowers or flowers used in most residential gardens are pollinated by bees, wasps, and other insects and therefore are not widely capable of producing allergic disease.

What is the Role of Heredity in Allergy?

Like baldness, height and eye color, the capacity to become allergic is an inherited characteristic. Yet, although you may be born with the genetic capability to become allergic, you are not automatically allergic to specific allergens. Several factors must be present for allergic sensitivity to be developed:

The specific genes acquired from parents.

The exposure to one or more allergens to which you have a genetically programmed response.

The degree and length of exposure.

A baby born with the tendency to become allergic to cow's milk, for example, may show allergic symptoms several months after birth. A genetic capability to become allergic to cat dander may take three to four years of cat exposure before the person shows symptoms. These people may also become allergic to other environmental substances with age.

On the other hand, poison ivy allergy (contact dermatitis) is an example of an allergy in which hereditary background does not play a part. The person with poison ivy allergy first has to be exposed to the oil from the plant. This usually occurs during youth, when a rash does not always appear. However, the first exposure may sensitize or cause the person to become allergic and, when subsequent exposure takes place, a contact dermatitis rash appears and can be quite severe. Many plants are capable of producing this type of rash. Substances other than plants, such as dyes, metals, and chemicals in deodorants and cosmetics, can also cause a similar dermatitis.

Also learn more about what causes allergies from the American College of Asthma, Allergy & Immunology (ACAAI).